The present invention relates generally to a fail-safe system in an electronic engine control system, which allows the engine to continue running even after one of the components of the electronic engine control system fails. More particularly, the invention relates to a back-up system for an engine coolant temperature sensor, which is adapted to produce a substitute signal approximately representative of the engine coolant temperature when failure of the engine coolant temperature sensor is detected.
As is well known, an engine coolant temperature is an important and fundamental control parameter in the electronic engine control processes, such as fuel injection control, idling speed control, fast idle control and so forth. Generally, the engine coolant temperature is detected by an engine coolant temperature sensor which produces a signal having a value corresponding to the engine coolant temperature. The engine coolant temperature sensor is installed in a water jacket surrounding the engine cylinders. For example, when a thermistor-type engine coolant temperature sensor is used, the output voltage of the sensor at -40.degree. C. is about 4V and at 120.degree. C. is 1V. In electronic engine control systems, the engine coolant temperature sensor output is fed to a microcomputer via an analog-to-digital converter (hereafter referred to as A/D converter).
If the engine coolant temperature sensor fails or the wiring connecting the sensor to the microcomputer breaks, the values received by the microcomputer from the engine coolant temperature sensor will fall into an abnormal range. If left uncorrected, this would result in failure of the engine control system. Conventionally, the engine control system is provided with a fail-safe or back-up system in order to continue control system operation even when the engine coolant temperature sensor fails. In the conventional system, the engine control system is responsive to the abnormal values of the engine coolant temperature signal to set the engine coolant temperature parameter to a predetermined value which corresponds to a normal range of engine coolant temperature, e.g. 80.degree. C. While this back-up system can keep the engine control system operative, the control by the engine control system may not accurately correspond to engine operating conditions. In particular, under relatively cold engine conditions, such a back-up system can degrade engine start-up characteristics and drivability.